As a projection type display device or a direct-view type liquid crystal display device, there is known an image display device that displays video by spatially modulating light emitted from a light source (hereinafter referred to as a lamp) with a display device having a transmission or reflective optical modulation effect. An image display device using a display device having such an optical modulation effect has a problem of so-called backlight bleeding in dark portions in comparison with an image display device using a self-light emitting display element such as a CRT. Backlight bleeding is a phenomenon in which black-displaying pixels are observed as glowing in a pitch dark environment when the illumination is turned off.
As one of the methods of suppressing the backlight bleeding and thereby improving the contrast of a display image, there has been proposed a method in which the emission brightness of the lamp and the diaphragm disposed in a light path from the lamp to the display device are changed dynamically in accordance with the scene of input video so as to control the amount of light that enters the display device. For example, the method is disclosed in Patent Document 1.
Patent Document 1 discloses that the contrast is improved by detecting the characteristics of the input video signal, and controlling the power of the lamp according to the detected characteristics. As the power of the lamp is changed according to the input video signal, the rotation speed of a cooling fan is controlled to maintain the temperature of the lamp within a range where the reliability of the lamp can be ensured.    Patent Document 1: JP H11-337897A